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Home , Solar dynamo

Solar dynamo models and magnetic flux emergence

Laurène Jouve Institut de Recherche en Astrophysique et Planétologie Toulouse-France

In collaboration with Sacha Brun, Rui Pinto and Guillaume Aulanier The magnetic Flux emergence observed Yohkoh soft Xray images over a whole solar cycle at different wavelengths (1991-2001) (Georgoulis et al. 2004, Schmieder et al. 2004)

Butterfly diagram (observed radial field) (D. Hathaway) Complex active regions with mixed helicity (Chandra et al, 2010)

2 Theory: the dynamo mechanism

Dynamo mechanism: process through which motions of a conducting fluid can permanently regenerate and maintain a against its ohmic dissipation It consists of the regeneration of both poloidal and toroidal fields

Poloidal Toroidal Toroidal Poloidal

Ω effect α effect Babcock-Leighton effect

3 Schematic theoretical view of the solar cycle

1: magnetic field generation, self-induction 4: Parker instability 2: pumping of mag. field 5: emergence+rotation or 6: recycling through α-effect or 2’: transport by meridional flow 7: emergence of twisted bipolar 3: stretching of field lines through Ω-effect structures at the surface 4

Tachocline helps for field organisation

• Large-scale magnetic field built by the dynamo • Stored in the sub-adiabatic layer at the base of the

Browning et al 2006

Ghizaru et al 2010 Racine et al 2011

5 Buoyant loops are generated (even without a )

• Buoyant magnetic loops form and rise up quite coherently • They are intrinsically twisted by convective motions Nelson et al., 2011, 2014

Fan & Fang 2014

6 Simulations of individual buoyant loops

Jouve, Brun & Aulanier 2013

• Creation of negative toroidal field through the Omega-effect is even more efficient.

• Same kind of asymmetries.

• Strong deformation of the rising structure because of convective motions.

• Similar rise time for the same field strength.

Convective layer

Movie created with SDvision@CEA 7 Simulations of individual buoyant loops

Convective upflows and downflows control the rise velocity of the loop and modify the morphology of emerging regions Latitude

Longitude

8 Emerging regions!

• Faster rise at high latitudes, thus stronger fields: effect of rotation • Shift in longitude: effect of • Tilt angle higher at high latitudes (in agreement with observations)

Merging • Origin of complex active regions with mixed helicity or polarity? (Chandra et al 2010, Toriumi et al 2014, Fang & Fan 2015)

No Merging

• Loop interactions (Jouve, Brun & Aulanier, in prep.) 9 Schematic theoretical view of the solar cycle

1: magnetic field generation, self-induction 4: Parker instability 2: pumping of mag. field 5: emergence+rotation or 6: recycling through α-effect or 2’: transport by meridional flow 7: emergence of twisted bipolar 3: stretching of field lines through Ω effect structures at the surface 10

A mean-field dynamo model

• Mean-field induction equation

• Poloidal/toroidal decomposition

• 2 coupled PDEs

Standard model: single- celled meridional circulation Cyclic field

Butterfly diagram close to observations

11 Link to the external : coupling codes

Kinematic dynamo Corona, Pinto et al, 2011

The solar wind code is fed with the surface magnetic field produced by the dynamo code

Solar wind responds to cyclic variations of the external field

12 Link to the external Sun: coupling codes

Solar wind speed at 15R in agreement with observations of Ulysses (Wang & Sheeley, 2006)

Poster 3.11 (Pinto)

13 Connections with

• Can we predict the next maximum?

Actual maximum in April 2014

Pesnell 2012 14 Connections with space weather

• Inspired from weather forecasting on Earth: -based models and observations combined through data assimilation

• Dynamo models+ observations of surface flows and mag. fields (Kitiashvili et al. 2008, Jouve et al. 2010, Hung et al. 2015) Poster 1.9 (Hung)

Credit: E. Kalnay

15 Conclusions

• Tools to study dynamo action in and interfaces between key regions: - observations - simplified models of a global mechanism - more complex and realistic models of particular steps of the dynamo loop - coupling codes which deal with different regions

• We have seen that: - the tachocline region may help to organise the magnetic field - 3D simulations of loops rising on a convective zone produce bipolar structures with properties in agreement with observations - data assimilation starts to be applied to for cycle predictions

• Several open questions remain: - is the tachocline a major player in the dynamo mechanism? - where do the active regions and their complexity actually come from? - what are the best ways to connect simulations of the solar interior/exterior?

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